The present invention relates to substantially completely ferritic stainless steel having improved cold-rolled surface quality by substantially eliminating the formation and precipitation of oxides and titanium nitrides during casting. More particularly, the invention relates to ferritic stainless steel flat rolled products having good surface quality by stabilizing with controlled amounts of both titanium and niobium, and in some embodiments having improved elevated temperature oxidation resistance and strength compared to conventional type 409. Processing of the ferritic stainless steel is also provided.
Ferritic stainless steels have found increasing acceptance in automotive vehicle components such as exhaust systems, emission control systems and the like. Such end uses require steels having good high temperature strength and resistance against oxidation and corrosion. In comparison to austenitic stainless steels, ferritic stainless steels have inherent advantages for applications at elevated temperature. Particularly, ferritic stainless steels have a lower coefficient of thermal expansion, higher thermal conductivity and better resistance to oxidation during thermal cycling. When compared to austenitic steels, however, the ferritic stainless steels have certain disadvantages such as inferior strength at elevated temperature, welding and forming characteristics.
Steels for the automotive exhaust systems must meet certain specific requirements for mechanical properties, corrosion resistance, oxidation resistance, and elevated temperature strength as mentioned above. Extensive development work has gone into such alloys to meet these demands. A commonly used grade, type 409, is a chromium ferritic stainless steel having nominally 11% chromium and is stabilized with titanium. Such an alloy was developed in the 1960's, as disclosed in U.S. Pat. No. 3,250,611, issued May 10, 1966. Higher chromium steels such as on the order of 18% chromium are known to have greater oxidation and corrosion resistance and are also used for automotive exhaust systems. Today's exhaust system material requirements include higher temperature service, ability to be deformed severely, and better surface quality. In addition to hot strength and continuous and cyclic thermal oxidation resistance, such steels should have improved formability, such as for tubular manifolds, be weldable and be capable of being produced in thinner gauge.
It has been suggested by others in the art that additions of titanium, or niobium, or both can improve certain properties of ferritic stainless steels. U.S. Pat. No. 3,250,611, mentioned above, discloses a ferritic steel having 10 to 12.5% chromium and stabilized with 0.2 to 0.75% titanium. The alloy was specifically developed for automotive exhaust systems and later became known as Type 409. Elongations of such T409 averaged about 24%, surface quality was poor, however, the alloy performed extremely well in mufflers and exhaust pipes.
Attempts have been made by others to improve the surface appearance and minimize roping by the addition of niobium to ferritic stainless steels. U.S. Pat. No. 3,936,323, issued Feb. 3, 1976 and No. 3,997,373, issued Dec. 14, 1976 disclosed a steel having 12-14% chromium and from 0.2 to 1% niobium which is annealed and cold-rolled to a reduction of at least 65%. U.S. Pat. No. 4,374,683, issued Feb. 22, 1983, discloses a 12 to 25% chromium ferritic stainless steel containing copper and 0.2 to 2% niobium which when processed in a specific manner exhibits good surface appearance and good formability without roping.
It is also known, however, that niobium alone cannot be used as a stabilizer when the steel is to be fabricated to a welded product. Niobium contributes to weld cracking, however, it is known that adding at least 0.05% titanium in niobium stabilized ferritic stainless steels does substantially eliminate weld cracking.
Other ferritic stainless steels have been developed containing both titanium and niobium with or without other stabilizing elements. British Pat. No. 1,262,588 discloses such a steel for automotive exhaust components, wherein the chromium-titanium-aluminum steel contains at least 0.3% of titanium, zirconium, tantalum, and/or niobium for improved oxidation resistance at elevated temperatures. Another ferritic steel developed for improved creep resistance and oxidation resistance contains 0.1 to 1% niobium and titanium based on the amount of carbon and nitrogen up to an amount of 1% for a chromium-aluminum alloy disclosed in U.S. Pat. No. 4,261,739, issued Apr. 14, 1981.
U.S. Pat. No. 4,286,986, issued Sept. 1, 1981, discloses a process for producing a creep resistant ferritic stainless steel having a controlled chemistry including 0.63 to 1.15% effective niobium which may be replaced by tantalum. This steel is then annealed at a temperature of at least 1900.degree. so as to improve creep strength.
Although it is generally known that titanium stabilized ferritic steels cannot be readily brazed with filler material such as oxygen free copper and nickel based alloys, a stabilized ferritic stainless steel composition which is wettable by conventional brazing materials is disclosed in U.S. Pat. No. 4,461,811, issued July 24, 1984, wherein the 10.5 to 13.5% chromium steel having up to 0.12% titanium, and up to 0.12% aluminum plus titanium is stabilized with titanium, tantalum and niobium in accordance with a stabilization formula.
It is known that the oxidation resistance of stainless steels can be improved as a result of the silicon content, as disclosed is an article in Oxidation of Metals, Volume 19, 1983, entitled "Influence of Silicon Additions on the Oxidation Resistance of a Stainless Steel" by Evans, et al. Such silicon containing stainless steels are known to be stabilized in order to improve certain properties. For example, U.S. Pat. No. 3,759,705, issued Sept. 18, 1973, discloses a 16 to 19% chromium alloy having 0.5 to 1.4% silicon, 1.6 to 2.7% aluminum, 0.15 to 1.25% niobium and 0.15 to 0.8% titanium. The alloy is said to have improved elevated temperature oxidation resistance and good cold formability.
U.S. Pat. No. 3,782,925, issued Jan. 1, 1974, discloses a 10 to 15% chromium ferritic stainless steel having small amounts of aluminum, silicon, titanium and one of the rare earth metals to provide a steel having improved oxidation resistance and an adherent oxide scale.
Another ferritic stainless steel having improved ductility and cold formability contains 13 to 14% chromium, 0.2 to 1% silicon, 0.1 to 0.3% aluminum and 0.05 to 0.15% titanium, as disclosed in U.S. Pat. No. 3,850,703, issued Nov. 26, 1974.
It is also known that niobium has a beneficial effect on the creep strength of ferritic stainless steels. An article entitled "Influence of Columbium on the 870.degree. C. Creep Properties of 18% Chromium Ferritic Stainless Steels" by Johnson, SAE, February, 1981, discloses the improvement in such steels for automotive exhaust systems, particularly with the combination of approximately 0.5% free columbium (niobium) and a high final annealing temperature.
Attempts have been made to improve the weldability as well as the cyclic oxidation resistance and creep strength at elevated temperature for ferritic stainless steels. U.S. Pat. No. 4,640,722 issued Feb. 3, 1987 discloses a steel containing 1 to 2.5% silicon, greater than 0.1% niobium uncombined and up to 0.3% niobium combined and further stabilization with titanium, zirconium and/or tantalum in accordance with a stoichiometric equation.
Japanese Pat. No. 20,318 (published in 1977) discloses ferritic stainless steels containing titanium and niobium in amounts based on the carbon and nitrogen content of the steel as well as 0.5 to 1.5% silicon in a 4 to 10% chromium steel to improve weldability and cold workability.
Although Type 409 ferritic stainless steel has remained the preferred alloy of the automotive industry for exhaust systems and other high temperature service, the titanium and carbon levels have been reduced resulting in improved ductility and surface quality. In the 1980's the demand for manufacturing tubular exhaust components requires even lower carbon and titanium levels in an effort to further improve ductility, fabricability and weldability, however, such steels provide lower yield strengths, hardness and tensile strength. The automotive industry is further placing more stringent surface appearance requirements on such ferritic steels.
Titanium used to stabilize alloys such as Type 409, for fabricating automotive mufflers, pipes, manifolds, catalytic converters, has an extremely high affinity for nitrogen and oxygen and readily combines with these elements during melting, refining and casting to form and precipitate the nonmetallic oxides and intermetallic TiN. Such precipitates coalesce into large chunks or clusters and float to the surface of the cooling molten metal in the mold because they are less dense than the liquid metal. Upon freezing, the oxides and TiN clusters are trapped in or near the surface of the cast slabs. When this occurs, costly slab grinding and coil grinding is required to minimize rolling these clusters into detrimental and rejectable surface defects that reduce product yield and increase scrap and rework of the coils.
It has been suggested in the prior art that mechanical dams and filters may be used to trap intermetallic and nonmetallic compounds in molten steel. Such devices are costly, cumbersome and do not always work.
Additional processing steps such as slab grinding and coil grinding improve the surface condition but do not eliminate the so-called "open surface defect". Furthermore, the open surface defect worsens as the sheet or strip material is rolled to lighter gauges. An "open surface defect" appears as a gray or dark streak parallel to the rolling direction in the hot rolled band, which streak appears to have been rolled into the coil surface. The relative length and width of each defect in the hot rolled band is a good indication of the relative size of the clusters in the steel prior to rolling. Visual examination reveals numerous cross-breaks in the defect which indicate that the open surface defect is composed of material having a lower ductility than the steel matrix along with which it is rolled.
During casting into ingots, the stream from the ladle may react with air to form oxides and titanium nitride clusters that tend to concentrate near ingot surfaces. This condition, sometimes called "bark", is highly objectionable and must be removed by conditioning, such as grinding, to produce a saleable product.
There still exists a need for a ferritic stainless steel alloy suitable for high temperature service which does not exhibit the open surface defects of titanium-bearing stainless steels. Such steels should be capable of being produced in light gages on the order of less than 0.015 inch without surface defects or holes. The steel and the method of producing the same should substantially eliminate the formation of intermetallic and nonmetallic titanium precipitates at or near the surface of ingots or continuously cast slabs in order to provide a cold-rolled sheet or strip product which is substantially free of the open surface defect. Furthermore, such ferritic stainless steel should be able to be produced by lower cost processes which eliminate the need for additional slab or coil grinding procedures and which permit rolling to thinner gauges as a result of eliminating the formation of the titanium nitride precipitates. Any alloy produced should be at least comparable to the Type 409 alloy in use in the automotive exhaust systems in terms of fabricability, and oxidation and corrosion resistance.